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Women are at an increased risk for breast cancer for more than 20 years after giving birth, according to a new study published in the Annals of Internal Medicine. This, the authors noted, means that a recent childbirth should be viewed as a risk factor for young women.

The authors studied data from 15 cohort studies for their research, including 9.6 million person-years of follow-up.

“We used combined data from 15 cohort studies to assess breast cancer risk after childbirth,” wrote Hazel B. Nichols, PhD, University of North Carolina Gillings School of Global Public Health in Chapel Hill, North Carolina, and colleagues. “The use of international, prospective data offers a new opportunity to assess the strength and duration of associations between recent childbirth and breast cancer risk while considering the effect of such factors as breastfeeding and family history of breast cancer.”

All participants were younger than 55 years old. The mean age when patients first entered the study was 41.8 years old. Overall, the researchers counted more than 18,000 cases of breast cancer among the more than 889,000 women they examined. A woman’s hazard ratio (HR) was found to be at its peak roughly five years after their most recent childbirth. Roughly 24 years after giving birth, the risk goes back to being the same as a woman who has not given birth.

Among women who have not given birth, there were 620 breast cancer cases per 100,000 women at age 45, 1,252 at age 47.5 and 1,955 at age 50. For women who have given birth in the last 3-6.9 years, there were 661 breast cancer cases per 100,000 women at age 45, 1,422 at age 47.5 and 2,202 at age 50.

“This corresponds to 41, 170, and 247 excess cases of breast cancer per 100 000 women at each respective age for women whose most recent birth was 3 to 6.9 years before, compared with nulliparous women,” the authors explained.

When women who had a family history of breast cancer gave birth, it led to an increased risk for breast cancer. Choosing to breastfeed one’s children was not found to affect the breast cancer risk of a woman who has recently given birth.

“Women and health care professionals should take these factors into account when considering individual risk profiles for breast cancer,” the authors concluded.